The present invention relates to a method of producing magnetron anode for use in household microwave oven.
One of the conventional magnetron anodes for use in household microwave oven has, as shown in FIG. 1, a circular steel cylinder 1 to the inner peripheral surface of which secured is a copper anode, i.e. a circular copper cylinder 2 which in turn is provided at its inside with a plurality of radial copper vanes 3. Also, a process as shown in FIG. 2 has been proposed, for example, in the specification of U.S. Ser. No. 819,056 filed July 26, 1977, now U.S. Pat. No. 4,200,217, as a method of producing the magnetron anode of the type described above.
In order to clarify the drawback of the prior art, a description will be made hereinafter as to the conventional method with reference to FIGS. 2a to 2f.
As shown in FIG. 2a, a steel cylinder blank material 21 having a wholly or partially tapered inner peripheral surface and a disc-shaped copper member 22 are used as the blank materials for the circular steel cylinder and the copper anode, respectively. After cleaning the inner peripheral surface A of the steel cylinder blank material 21 and the outer peripheral surface B of the disc-shaped copper member 22 as shown in FIG. 2a, the steel cylinder blank material 21 is placed in a die 25 and the disc-shaped copper member 22 is fitted to the inside of the steel cylinder blank material 21, as shown in FIG. 2b. Then, a plastic working is effected on the disc-shaped copper member 22 in the steel cylinder blank material 21, by means of a shaping punch 24 and a back-up punch 26, so that the outer peripheral surface of the disc-shaped copper member 22 may be closely and tightly fitted to the inner peripheral surface of the steel cylinder blank material 21, as shown in FIG. 2c. This step for obtaining the close and tight fit will be referred to as "compaction", hereinafter. The composite member as shown in FIG. 2e is then subjected to a press work by a shaping punch 27 having the illustrated shape, after a preheating, so that a plurality of vanes 23 as shown in FIG. 2e are shaped. This step will be referred to as vane-shaping, hereinafter. During this vane-shaping, the steel cylinder blank material 21 is naturally raised, due to the presence of the tapered interface. Finally, the half-finished article as shown in FIG. 2e is subjected to an after processing and then to a diffusion annealing, so as to become a magnetron anode having a construction as shown in FIG. 2f. The copper anode consisting of a circular copper cylinder 2 and a plurality of copper vanes 3 integral therewith is thus attached to the inner peripheral surface of the circular steel cylinder 1 at a high reliability.
As has been stated, the conventional method requires the use of a material having tapered inner surface, as the blank for the circular steel cylinder. Usually, this tapered inner surface is processed by means of a lathe or the like, at a cost of considerably long working time. This inconveniently hinders the mass production of the magnetron anode.
It is therefore expected that the mass-produceability of the magnetron anode can be remarkably improved if the taper of the inner peripheral surface of the steel cylinder blank material is dispensed with.
It is impossible, however, to shape the copper anode vanes having a complicated shape as shown in FIG. 1 with a required precision and a good compaction to the inside of the circular steel cylinder, by the described conventional method, if the steel cylinder blank material lacks the tapered inner surface, i.e. if a straight steel cylinder blank material is used as the blank material for the circular steel cylinder. In other words, it has been essential in the conventional method to wholly or partially taper the inner peripheral surface of the steel cylinder blank material.
The described conventional method involves another problem. In this conventional method, the copper blank is subjected to a plastic working while it is embraced by the steel cylinder blank material. Therefore, during the pressing under room temperature or a secondary plastic working which is effected at a high temperature, the steel cylinder blank material 1' is likely to be deformed plastically by the shaping force exerted on the copper blank 2', as shown in FIG. 3, or even made to burst by such a force.
To avoid this, it has been necessary to previously quench-harden the steel cylinder blank material. The adoption of this quench-hardening requires a considerably long time, not only for the heating and quenching, but also for the elimination of oxidation film and distortion in shape which is caused as a result of the quenching, which also hinders the mass production of the magnetron anode.